1. Field of the Invention
The present invention relates to exposure of a photosensitive substrate in the preparation of semiconductor devices and liquid crystal display devices, and more particularly a method of determining exposure conditions in an exposure apparatus, especially through the evaluation of dimensions of a pattern formed on the photosensitive substrate by said exposure apparatus, and a method of inspecting the precision of the exposure apparatus.
2. Related Background Art
The conventional exposure apparatus of this kind is provided, for exposing a photosensitive substrate having a layer of photoresist of a predetermined thickness (1-5 .mu.m) to the image of a pattern (geometrical pattern consisting of light-transmitting areas and opaque areas) formed on an original plate called a mask or a reticle, there is provided an illuminating system for illuminating said mask or reticle from above, with uniform distribution and with substantially constant light intensity for a predetermined period, or an illuminating system for emitting plural pulses from a pulsed laser until a predetermined integrated light amount is obtained. In either case, control is made in such a manner that the pattern of the reticle can be printed onto the photoresist layer with an optimum amount of exposure.
Such exposure control is conducted under very strict conditions such as .+-.3%, in order to control the pattern width formed on said photoresist layer, with sufficient precision.
On the other hand, the width of the pattern formed on the photoresist layer is significantly influenced by the mechanical distance between mask and the photosensitive substrate in case of proximity exposure system, or by the distance between the photosensitive substrate and the projection optical system, or the focusing error, in case of projection exposure system.
The optimum exposure condition for the photosensitive substrate, particularly the focus condition and amount of exposure, have been determined by a trial exposure on the photosensitive substrate and comparison of the width of developed linear pattern, measured by an optical microscope or a line width measuring apparatus, with the design line width, or by a phenomenon that the line width becomes smallest under certain conditions.
For example, a step-and-repeat exposure apparatus employs an exposure system of stepping the wafer with respect to the reticle, in order form shot areas in matrix shape on said wafer. In the trial exposure, therefore, there is considered a method of varying the amount of exposure (for example the period of shutter aperture) while maintaining a constant focusing in the lateral (x) direction in the matrix arrangement of the shot areas on the wafer, and varying the focusing (for example by 0.25 .mu.m) while maintaining a constant exposure in the vertical (y) direction in said matrix arrangement.
After the image development, the line width in the photoresist pattern in the shot areas is directly measured, and the optimum exposure conditions are determined by the focusing condition in a shot area in which the line width is smallest among the shot areas of same exposure, and the amount of exposure giving a predetermined line width among the shot areas with said focusing condition.
The conventional technology explained above has been associated with a drawback that the processing speed is extremely slow, since, after the transfer of a pattern formed on a test reticle to a photoresist layer formed on a wafer, the distances of parallel edges of thus formed pattern have to be measured with an optical microscope or an exclusive measuring apparatus. Particularly in case of observing the photoresist pattern with an optical microscope coupled with an ITV camera, said microscope has to be precisely focused, causing severe fatigue on the part of operator. The exclusive measuring apparatus can achieve relative precise line width measurement, but such apparatus has to be made available.
There is an additional drawback: that the error of the pattern formed on the test reticle is directly reflected on the measured width of the photoresist pattern.
In general, in the projection exposure apparatuses, the best focus plane of the projection is scarcely flat in the submicron precision over a field as large as 15.times.15 mm or 20.times.20 mm (corresponding to a shot area), but involves a curvature or an inclination of the image plane close to one micron, between the center and the peripheral part of the field.
However, as the depth of focus of the projection lens is as small as .+-.1 .mu.m, it is necessary to determine the best imaging plane in order to obtain average focus state over the entire field. For this reason linear patterns or other marks are provided in plural positions on the test reticle in order to obtain such patterns or marks for line width measurement in plural points at the center and peripheral area of each shot area. In such case the error in the line width in such marks and patterns are directly reflected in the line width measurement. It is therefore difficult to precisely determine the image plane curvature or inclination, so that the focusing conditions can only be determined in unprecise manner.
Also the line width measurement of the pattern formed in the photoresist layer can be achieved, in addition to the above-explained method of observation with an ITV camera and measurement of the dimension between the pattern edges through the analysis of thus obtained image signal, by a method of scanning the photoresist image on the wafer with a laser beam spot of several microns in width and determining the width from the correlation between the scattered light intensity from the pattern edges and the scanning position of the laser beam spot, or by a method (SEM method) of scanning the photoresist image with an electron beam spot and determining the width from the correlation between the secondary electron beam intensity from said photoresist image and the scanning position of the electron beam.
Also in order to check the precision of the exposure apparatus, there has been employed a method of employing a test reticle with special vernier patterns, effecting double exposure with a displacement of the wafer in such a manner that the main vernier scale and the auxiliary scale mutually overlap on the wafer and observing the developed photoresist patterns of said vernier scales under a microscope, or another method of printing two parallel linear patterns (constituting a diffraction grating) and measuring the distance of said two linear patterns with the alignment sensor of the exposure apparatus, as disclosed in U.S. Pat. No. 4,803,524.
Among three methods mentioned above, the method employing the ITV camera and the method employing laser beam spot are limited in resolving power, for example by simply detecting the width of a linear photoresist pattern, due to the presence of diffraction. Particularly the latter method is unable to measure the line width smaller than the spot size, which cannot be reduced below the limit of diffraction. Although the former method is simple, it is associated with a drawback that the resolving power and the precision of measurement is directly influenced by the performance of the objective lens of the microscope for observing the photoresist image.
Also the electron beam (SEM) method has been associated with drawbacks of low throughput of measurements and of requiring expensive apparatus, as the wafer specimen has to be measured in a high vacuum chamber, requiring a long evacuating time.